This disclosure relates generally to heat exchangers with features directed to various innovations including ones relating to the gas turbine recuperators.
The recuperation of the gas turbine engine has been proven to increase thermal efficiency. However, the technical challenges associated with surviving the severe environment of a gas turbine exhaust while meeting the equally severe cost challenges has limited the number of viable products. A gas turbine recuperator is typically exposed to a thermal gradient of up to 600 degrees C., pressures of 3 to 16 bar, and may operate at a gas temperature of over 700 degrees C. Moreover, developers of advanced recuperated Brayton (gas turbine) systems are considering applications with pressures of up to 80 bar and temperatures ranging to 1000 degrees C.
The successful design must tolerate severe thermal gradients, and repeated thermal cycling, by allowing unrestricted thermal strain. The structural requirements to manage very high pressures tend to work against the normal design preferences for structural flexibility, which is important to tolerating large and rapid thermal transients.
Child, Kesseli, and Nash (U.S. Pat. No. 5,983,992) describe a flexible heat exchanger design as shown in FIGS. 1A-1C. This design is composed of stamped parting sheets A, B, each formed with “substantially S-shaped” raised flanges C, D. These stamped hoops form an integral manifold in the plate. When welded cell to cell, the stack of manifolds becomes a flexible bellows-like structure. This feature represents the principal novelty of this prior art design over heat exchangers embodying a more rigid structure. While the flexibility of the manifold represents an advantage in environments of high thermal-induced strain, the thickness of the sheet and the manifold geometry limits its capacity for pressure. The inventors state that the light gauge sheet metal construction is critical to the performance and integrity of this design and superior to other designs employing edge bar or closure bar construction.
As exemplified by U.S. Pat. No. 4,073,340 to Garrett, other traditional manufacturers have produced heat exchangers formed of individual cells, brazed together employing stamped edge conditions and integral cut-out manifolds cut-out from the parting plate, principally similar to Child et al. (U.S. Pat. No. 5,983,992). FIGS. 2A and 2B illustrate the heat exchange apparatus of Garrett and shows stamped formed edge sheets E and manifold cutouts F and G. The complete heat exchanger core of this configuration is formed by coating the various elements with braze alloy, stacking the plates and secondary fin surfaces, and brazing the complete assembly in a furnace. Due to the sturdy edge bars, this design construction is likely to tolerate considerably higher pressures than the apparatus of Child et al. (U.S. Pat. No. 5,983,992). However, due to the monolithic structure formed as all contacting plate and fin surfaces are brazed, the rigid heat exchanger construction is prone to stress cracking caused by repeated thermal cycling.
British Patent No. 1,197,449 to Chausson shows a formed header like Child et al. (U.S. Pat. No. 5,983,992) and Garrett (U.S. Pat. No. 4,073,340) and the raised sheet metal manifold integral with the parting plates. Referring to FIG. 3, there appears the heat exchanger of GB1,197,449, which has a formed dish-shaped edge K, a high-density fin M between the parting plates, communicating with the formed manifold cutout L, configured to carry the first fluid. The second fluid, flowing on the outer surface of the parting plates passes through high-density fin matrix elements N and O, configured to carry the second fluid. The high-density fin matrix elements N, O are brazed to the parting plates, but not to one another, in a manner similar to Child et al. (U.S. Pat. No. 5,983,992). In addition, as with the device of Child et al., the construction is of light gauge sheet metal and best suited for low to moderate pressures.
Lowery (British Patent No. 1,304,692) discloses a cellular heat exchanger concept as shown in FIG. 4. Like Child et al. and GB1,197,449, this design uses a unit cell with light gauge external fin elements R and S bonded to the outside of an envelope forming a flow path for a first fluid, with internal passages inside the envelope forming passages for a second fluid. Also, as with the devices of Child et al. and GB1,197,449, the fin elements R and S of neighboring cells bear upon one another at crests T. A unique feature of this design relates to the heavy “pressings” forming the passages of the second fluid. These heavy pressings located in a hot gas stream tend lag in thermal response and consequently are prone to buckle when exposed to high temperature and steep thermal gradients. This design is most suitable for lower temperature air-water “radiator” applications.
U.S. Pat. No. 3,460,611 to Folsom et al. describes a plate-fin heat exchanger incorporating formed parting plates and strip fin. Quoting from this specification, “These parts are bonded or soldered together to make an integral unit or module and before that unit is incorporated in a stack or modules it conveniently may be tested and proven without leaks or cause to attain that condition.” See Folsom et al. at column 2, lines 51-55. See also claims 1 through 6 of U.S. Pat. No. 6,305,079 to Child et al. The heat exchange cell of Folsom et al., like that of Child et al, has formed lands around the perimeter. The apparatuses of Folsom et al. and Child et al. both incorporate formed lands around the header, thereby creating a cell not suitable for high internal pressure. Also, Folsom's formed semi-circular manifold requires an additional welding operation to attach the cell to a pipe or collector.
Based upon the foregoing limitations known to exist in plate-fin heat exchangers, it would be beneficial to provide a heat exchanger having a rigid manifold section capable of operation at elevated pressure, connecting to a light gauge, flexible sheet metal structure imposing limited mechanical constraints on and between neighboring cells.